The immune response includes both a cellular and a humoral response. The cellular response is mediate largely by T lymphocytes (alternatively and equivalently referred to herein as T cells), while the humoral response is mediated by B lymphocytes (alternatively and equivalently referred to herein as B cells). Lymphocytes play a number of crucial roles in immune responses, including direct killing of virus-infected cells, cytokine and antibody production, and facilitation of B cell responses. Lymphocytes are also involved in acute and chronic inflammatory disease; asthma; allergies; autoimmune diseases such as scleroderma, pernicious anemia, multiple sclerosis, myasthenia gravis, IDDM, rheumatoid arthritis, systemic lupus erythematosus, and Crohn's disease; and organ and tissue transplant disease, e.g., graft vs. host disease.
B lymphocytes produce and secrete antibodies in response to the concerted presentation of antigen and MHC class II molecules on the surface of antigen presenting cells. Antigen presentation initiates B cell activation through the B cell receptor (BCR) at the B cell surface. Signal transduction from the BCR leads to B cell activation and changes in B cell gene expression, physiology, and function, including secretion of antibodies.
T cells do not produce antibodies, but many subtypes of T cells produce co-stimulatory molecules that augment antibody production by B cells during the humoral immune response. In addition, many T cells engulf and destroy cells or agents that are recognized by cell surface receptors. Engagement of the cell surface T cell receptor (TCR) initiates T cell activation. Signal transduction from the TCR leads to T cell activation and changes in T cell gene expression, physiology, and function, including the secretion of cytokines.
Identifying ligands, receptors, and signaling proteins downstream of TCR, as well as BCR, activation is important for developing therapeutic regents to inhibit immune response in inflammatory disease, autoimmune disease, and organ transplant, as well as to activate immune response in immunocompromised subjects, and in patients with infectious disease and cancer (see, e.g., Rogge et al., Nature Genetics 25:96–101 (2000); U.S. Pat. Nos. 5,518,911; 5,605,825; 5,698,428; 5,698,445; 6,013,464; and 6,048,706).